1. Field of the Invention
The present invention relates to a method of piercing and manufacturing seamless tubes comprising a piercing process wherein a solid billet as a material for the seamless tubes is made thinner at high processability.
2. Description of the Prior Art
The Mannesmann plug mill process or Mannesmann mandrel mill process is most widely used hitherto as a method of manufacturing seamless tubes. In these processes, the solid billet heated to the prescribed temperature through a heating furnace is pierced with a piercing mill into a hollow piece, which is rolled into a hollow shell by means of an elongator, e.g. rotary elongator, a plug mill or mandrel mill, by reducing mainly its wall thickness, then the outside diameter is reduced by means of a reducing mill such as a sizer or stretch reducer to obtain finished seamless tubes having the specified dimensions.
Technical contents of our prior invention disclosed in U.S. Pat. No. 4,470,282 characterized particularly in a piercing method of such manufacturing process of seamless tubes will be described hereinbelow.
In the prior invention, a feed angle .beta. (an angle which the roll axis makes with a horizontal or vertical plane of the pass line and a cross angle .gamma. (an angle which the roll axis makes with a vertical or horizontal plane of the pass line) of cone-shaped main rolls supported at both ends, and disposed in horizontally or vertically opposed relation with the billet/hollow piece pass line therebetween are retained in the following ranges, EQU 3.degree.&lt;.beta.&lt;25.degree. EQU 3.degree.&lt;.gamma.&lt;25.degree. EQU 15.degree.&lt;.beta.+.gamma.&lt;45.degree.
and disc rolls disposed in vertically or horizontally opposed relation between said main rolls with the pass line therebetween, are pressed against the billet and hollow piece during piercing operation.
The prior invention is substantially contradictory to a piercing principle of the Mannesmann process, in which piercing is effected by using a so-called rotary forging effect (Mannesmann effect), whereas in the prior invention, (1) occurrence of the rotary forging effect (Mannesmann effect) is restrained as much as possible, and (2) the circumferential shear deformation .gamma..sub.r.theta. or shear strain due to surface twist .gamma..sub..theta.l produced during piercing process is restrained as much as possible, to realize the metal flow equivalent or proportional to the extrusion process though being rotary rolling.
For this purpose, the piercing mill is constructed to enable the high cross angle and high feed angle piercing, the main rolls are made conical and instead of guide shoes, disc rolls are employed. As a result of killing thereby the rotary forging effect (Mannesmann effect) to restrain initiation of inside bore defects and, in particular, releasing shear stress field of the circumferential shear deformation .gamma..sub.r.theta. to restrain propagation of the inside bore defects, the tube making of so-called materials of poor workability, such as a high alloy, super alloy and the like, e.g., Inconel, Hastelloy, etc., not to speak of free cutting steel and stainless steel which has had no way but to rely on the Ugine-Sejournet extrusion process hitherto is becoming possible.
Also, in a continuously cast billet having a center porosity, tubes can be manufactured without producing micro bore defects, thus contributing largely to advantages of rationalization such as manufacturing costs and the like.
Problems to be Solved by the Invention
In general, longitudinal, radical and circumferential strains, .psi..sub.l, .psi..sub.r and .psi..sub..theta. in piercing may be represented by the following equations, where the outside diameter and length of the solid billet before piercing are designated as d.sub.o, l.sub.o and those and thickness of a hollow piece after piercing are designated as d, l and r: ##EQU1## here, .psi..sub.l +.psi..sub.r +.psi..sub..theta. =
Though, by usage, indexes of piercing ratio and expansion ratio are used, they do not represent the quantity of deformation accurately, but defined as, ##EQU2## expansion ratio d/d.sub.o, which are just criteria for the degree of deformation. Since their intuitional meanings are clear, however, they are often used as indexes for deformation and are also utilized in the following description.
Now, in the usual piercing, though the piercing ratio is only about 3.0.about.3.3 and the expansion ratio about 1.05.about.1.08, our prior invention was also based upon such common ranges.
Accordingly, if the piercing ratio or expansion ratio is increased excessively above this, the rotary forging effect is emerged excessively to cause severe circumferential shear stress field in piercing, leading to the inevitable inside bore defect formation whereby a double piercing method using two piercing mills has had to be employed.
That is to say, the billet should be bored with the first piercing mill, and with the second piercing mill the wall thickness was reduced by further elongation (in this case, the second piercing mill is called a rotary elongator) or by expansion of 30 to 50% (in this case, the second piercing mill is called a rotary expander).